This PR aims to resolve #3602 by replacing all pandas' df.append() calls by pd.concat() calls in alignment with the corresponding 1.4.0 Release Notes. Since there should be no functionality affected, no bug fixed, no interface changed or similar whatsoever, we only changed the according calls in plotly.express and plotly.figure_factory. To make sure we did in fact not change the intermediate results of these calls, we temporarily kept the old calls in, and asserted them equaling our new calls to pd.concat(). Locally all tests passed and after successful pipeline runs we removed these assertions and the old calls, to finally get rid of the FutureWarnings.

This is my first contribution to Plot.ly, so please let me know, if I can improve anything.

As of now the build-doc job keeps failing although I have the impression, that this is unrelated to my changes. Please let me know, if I can do anything to fix this.

Code PR

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